TransmissionBT (R5ARM)

Easy.
Transmission is designed for easy, powerful use. We’ve set the defaults to Just Work and it only takes a few clicks to configure advanced features like watch directories, bad peer blocklists, and the web interface.

Lean.
In separate benchmarks, Linux Format and Lacrocivious both found Transmission to use less CPU than any other GUI client. It even used less CPU than some non-GUI clients.